This application claims priority from PCT/JP01/09157, filed Oct. 18, 2001, and JP2000-318779, filed Oct. 19, 2000, incorporated herein by reference.
The present invention relates to a medium and method for preventing forgery of two-dimensional and three-dimensional objects such as passports, cards, security notes, gift certificates, pictures, public transportation tickets, and public betting tickets.
Conventionally, hologram has been used as an identification medium for preventing the forgery of cards, security notes and coupons. However, as the fabrication of hologram gets easier over time, forgery has become more and more common, and some of the cases of forgery, in particular those in connection with media for visual identification, are so undistinguishable from authentic ones that it is virtually impossible to tell them apart. Therefore, there is a need for new identification media.
Japanese patent laid open publication (kokai) No. 63-51193 discloses the use of high polymer cholesteric liquid crystal as an identification medium, and the light reflected therefrom is detected either visually or by using an optical identification device.
Cholesteric liquid crystal normally has a layered structure, and the axial directions of the molecules in each layer are parallel to each other as well to the plane of the layer. Each layer is slightly twisted relative to the adjacent layer so that a three-dimensional spiral structure is produced. This structure demonstrates the property to selectively reflect a circularly polarized light having a wavelength of xcex which is given by xcex=nxc2x7p where p is the depth of the layers for this axial direction to turn 360 degrees or the pitch, and n is the average index of refraction of each layer. Low polymer cholesteric liquid crystal is used for thermometers whose color changes with temperature because the pitch p distinctly changes with temperature as well as for display devices.
On the other hand, high polymer cholesteric liquid crystal is provided with a relatively fixed crystal structure so that the pitch p changes very little with temperature, magnetic field and electric field, and selectively reflects light of a certain predetermined color (wavelength). High polymer cholesteric liquid crystal can be prepared, for instance, by heating high polymer having a property to be formed into thermotropic cholesteric liquid crystal beyond the liquid crystal transition temperature to cause a liquid crystal structure to grow, and fixing the liquid crystal structure by rapidly cooling it.
For instance, when high polymer cholesteric liquid crystal that reflects red light having a prescribed wavelength xcexR is placed over material which absorbs visible light such as black paper, and random light such as sun light is projected thereon, the transmissive light is totally absorbed by the underlying black paper, and only left circularly polarized light having the wavelength of xcexR is reflected. It produces an attractive special color which may be called as cholesteric color. This light however can be shut off if a filter for blocking left circularly polarized light is placed thereon.
When high polymer cholesteric liquid crystal that reflects red light having a prescribed wavelength xcexR is placed over material which reflects visible light such as white paper, and random light such as sun light is projected thereon, the underlying white color becomes visible. If a filter for transmitting only left circularly polarized light is placed thereon, only the reflected left circularly polarized light having the wavelength of xcexR become visible as red light. If left circularly polarized light having the wavelength of xcexR instead of sun light is projected thereon, only the reflected light having the wavelength of xcexR becomes visible even without using a filter.
However, when reflected light is used for identification purpose, although the high polymer cholesteric liquid crystal demonstrates a unique cholesteric color, and is attractive in appearance, it cannot provide a generally agreeable appearance by itself, and has been considered inadequate as an identification medium of an object whose aesthetic appearance is important. The graphic shapes (graphic shapes and patterns) formed on the identification medium are also an important factor in identifying the object. In this regard also, a further improvement of identification media using high polymer cholesteric liquid crystal is desired.
The present invention was made in view of such problems of the prior art, and its primary object is to provide an identification medium which provides an improved identification capability, is highly difficult to illicitly duplicate, is highly attractive in terms of both shape and color, and is easy to fabricate, and a method for making such an identification medium.
According to the present invention, such an object can be accomplished by providing an identification medium for identifying an authenticity of an object either visually or optically by using an identification device, characterized by that: the identification medium comprises high polymer cholesteric liquid crystal which is patterned, and/or a graphic representation layer such as a figure and letter which underlies or overlies high polymer cholesteric liquid crystal, and by providing a method for making an identification medium for identifying an authenticity of an object either visually or optically by using an identification device, characterized by the steps of: applying high polymer cholesteric liquid crystal onto the object, onto a sticker adapted to be affixed to the object or onto a base for hot stamping onto the object, and patterning the high polymer cholesteric liquid crystal, and/or forming a graphic representation layer such as a figure and letter under or over the high polymer cholesteric liquid crystal.
This enhances the aesthetic appearance, and improves the capability to identify the object and prevent an illicit duplication of the object.
In particular, by patterning the high polymer cholesteric liquid crystal by etching or by using laser, the high polymer cholesteric liquid crystal can be accurately and easily patterned into a desired shape so that its aesthetic appearance and identification capability can be both improved. By printing on the high polymer cholesteric liquid crystal, the printed color shows on the printed part and the cholesteric color shows on the exposed surface of the high polymer cholesteric liquid crystal so that the freedom in design is enhanced. By thermally transferring the high polymer cholesteric liquid crystal according to a prescribed pattern, small batch production is facilitated. By partly modifying the high polymer cholesteric liquid crystal by laser, the high polymer cholesteric liquid crystal is provided with partly different optical properties, and its aesthetic appearance and identification capability can be both improved.